The present invention relates to a cryogenic vapor recovery process and system for removing condensable vapors from a gas stream by condensation. More particularly, the present invention relates to such a process and apparatus in which refrigeration for the condensation is provided by a cryogenic refrigerant and the refrigeration imparted to the gas stream by the cryogenic refrigerant is at least partly recovered. More particularly, the present invention relates to such a process and apparatus in which two condensers are operated out of phase so that one of the condensers is on-line and operating to condense the vapors out of the gas stream while the other condenser is off-line and is being defrosted prior to being brought on-line.
The prior art has provided cryogenic vapor recovery systems for removing condensable vapors contained within a gas stream. An example of such a system is disclosed in U.S. Pat. No. 5,291,751, in which condensable vapors contained within a gas stream are removed from the gas stream by a hybrid system. In this hybrid system, the gas stream first passes through one or more mechanical refrigeration stages and then is routed to two cryogenic refrigeration stages having cryogenic heat exchangers to condense the condensable vapors from the gas stream. The two cryogenic refrigeration stages are operated out of phase so that when one heat exchanger is fully loaded up with solid condensate, the operation is switched to the other heat exchanger, and vice-versa to allow each of the heat exchangers to operate as condensers while the opposite heat exchanger is defrosted. U.S. Pat. No. 3,967,938 shows a cryogenic vapor recovery system, specifically designed to condense gasoline vapors from a gasoline tank, which utilizes propane as the refrigerant.
Another example of a cryogenic vapor recovery system is illustrated in U.S. Pat. No. 5,291,738 in which the gas stream passes through a mechanical chiller, an economizing heat exchanger, a main condenser which is refrigerated by a liquid nitrogen loop, and a mist eliminator. The refrigeration imparted to the gas stream by the nitrogen loop is at least partly recovered in the economizing heat exchanger where heat is indirectly exchanged between the gas stream as it enters the system with the gas stream prior to its discharge from the system. The main condenser is defrosted by recirculating part of the gas stream after having been heated through a heater. Since the defrosting requires a system shut-down, duplicate systems would have to be operated in order to provide continuous operation. U.S. Pat. No. 5,291,738, as contrasted with the foregoing reference patents, recognizes that refrigeration is imparted to the gas stream and if the gas stream is not recirculated, the refrigeration is lost. However, since the refrigeration is recovered upstream of the condenser, freezing of the condensables can occur before the condensers to cause blockage of the system.
As will be discussed, the present invention provides a cryogenic vapor recovery system in which refrigeration from the gas to be processed is at least partly recovered in such a manner that the system is amenable to be designed for continuous operation without duplication of the entire system and in which the freezing of the condensables is constrained to occur within condensers used in connection with the system.